2009_Book_FoodChemistry

4.2 Monosaccharides 253
The cis-arrangement of the two adjacent
HO-groups in positions C-1 and C-2 of α-Dglucopyranose,
unlike its β-anomer, increases the
conductivity of boric acid. A borate complex is
formed which is a stronger acid than boric acid
itself (cf. Formula 4.10).
In Tollens ring formula, in the D-series, the α-anomer
has the hydroxyl group at C-1 on the right
and the β-anomer has this OH-group on the left.
In Haworth projections the HO-group of
α/β-anomers of the D-series usually occurs
below/above the pyranose or furanose ring
planes, while in the L-series the reverse is true
(cf. Formula 4.11).
(4.10)
equilibrium state varies greatly among sugars (cf.
Table 4.6).
The transition into the different hemiacetal
forms, called mutarotation, proceeds via the
open-chain carbonyl compound. The acid- and
base-catalyzed ring opening is the rate limiting
step of the reaction:
(4.12)
2,3,4,6-Tetramethyl-D-glucose reaches equilibrium
in benzene rapidly through the concerted
action of cresol and pyridine as acid–base
catalysts (Table 4.3). Bifunctional reagents, like
2-pyridone and benzoic acid, are especially
efficient acid–base catalysts in both polar and
nonpolar solvents:
(4.13)
Water can also be a bifunctional catalyst:
(4.11)
Each monosaccharide can exist in solution
together with its open chain molecule in a total
of five forms. Due to the strong tendency towards
cyclization, the amount of the open chain form is
greatly reduced. The contribution of the different
cyclic forms to the equilibrium state in a solution
depends on the conformation. An aqueous
D-glucose solution is nearly exclusively the two
pyranoses, with 36% α- and 64% β-anomer,
while the furanose ring form is less than 1%. The
(4.14)
Table 4.3. Mutarotation rate of 2,3,4,6-tetramethyl-Dglucose
(0.09 mol/l) in benzene
Catalyst
(kmin −1 ) k rel
– 7.8 ×10 −5 1.0
Pyridine (0.1mol/l) 3.7 ×10 −4 4.7
p-Cresol (0.1mol/l) 4.2 ×10 −4 5.4
Pyridine + p-cresol (0.1mol/l) 7.9 ×10 −3 101
2-Pyridone (0.1mol/l) 1.8 ×10 −1 2307
Benzoic acid (0.1mol/l) 2.2 28,205